The present invention is directed to an apparatus for scanning colored registration marks printed on a web by a multi-color printing press having a separate printing station for printing each of a plurality of colors.
Color printing presses are typically provided with at least four print stations through which a web, such as paper, sequentially passes. Each printing station includes a rotating printing cylinder that prints an image in a single color on the paper. The images printed by the printing cylinders must be properly aligned or registered so that each single-color image precisely overlays the other single-color images to form the desired multi-color image. To maintain proper alignment of the images, each printing cylinder must be maintained in a proper angular orientation with respect to the other printing cylinders.
Conventional printing presses include dynamic registration systems which maintain the proper registration of the printing cylinders during printing by detecting the relative position of a plurality of registration marks printed on the moving web in different colors, each registration mark being printed by one the printing cylinders. Any error in registration of the printing cylinders is determined by detecting a variation in the relative position of the registration marks on the web.
A prior art scanner has been used to detect different colored registration marks printed by the printing cylinders of a printing press. The prior art scanner directed a beam of light at the moving web on which the registration marks were printed and detected light reflected from the moving web. The scanner generated an analog reflectance signal having a magnitude based on the amount of reflected light. Since different colors reflect different amounts of light, the presence of a registration mark printed in a color different than the color of the paper web is detectable by detecting a change in magnitude of the reflectance signal.